Method and communication system for transmitting information with the aid of a multicarrier method

ABSTRACT

To transmit information with the aid of a transmit signal exhibiting a number of frequency-specific subcarriers from a first unit to a second unit via a transmission medium, the frequency-selective transmission characteristics of the transmission medium are determined in the first unit and then the subcarriers of the transmit signal are adapted to the transmission characteristics determined. All subcarriers of the transmit signal can be advantageously modulated with the same number of modulation levels as a result of which maximum utilization of the transmission resources of the transmission medium is achieved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present relates, generally, to a method and communication system fortransmitting information with the aid of a multicarrier method and, morespecifically, to such a method and system wherein maximum utilization ofavailable transmission resources of a transmission medium is achievedduring the transmission of information via the transmission medium whichhas frequency-selective transmission characteristics.

2. Description of the Prior Art

In wireless communication networks based on radio channels, especiallyin point-to-multipoint radio feeder networks (also called “radio in thelocal loop” or, respectively, “RLL”), a number of network terminatingunits are, in each case, connected to a base station (also called “radiobase station” or, respectively “RBS”) via one or more radio channels. Intelcom report No. 18 (1995), vol. 1 “Drahtlos zum Freizeichen” [Wirelessto the ringing tone] page 36, 37, for example, a wireless feeder networkfor the wireless speech and data communication is described. Thecommunication system described represents an RLL subscriber line incombination with a modern broadband infrastructure, e.g. “fiber to thecurb”, which can be implemented within a short time and without greatexpenditure instead of running wire-connected local loops. The networkterminating units RNT allocated to the individual subscribers areconnected to a higher-level communication network, for example to theISDN-oriented landline network, via the “radio channel” transmissionmedium and the base station RBS.

Due to the increasing spread of multimedia applications, high-bit-ratedata streams must be transmitted rapidly and reliably via communicationnetworks, especially via wireless communication networks or,respectively, via mobile radio systems, and high demands are made on theradio transmission systems which are based on a transmission medium“radio channel” which is susceptible to interference and difficult toassess with regard to the quality of transmission. A transmission methodfor transmitting broadband data streams, such as video data streams, isrepresented by, for example, the OFDM (orthogonal frequency divisionmultiplexing) transmission method based on a so-called multicarriermethod. In the OFDM transmission technology, the information to betransferred or, respectively, the data stream to be transferred isdivided or, respectively, converted to parallel form, to a number ofsub-channels or subcarriers within the radio channel. The information tobe transferred in each case being transmitted at a relatively low datarate but in parallel in relatively superimposed form. The OFDMtransmission technology is used, for example, in digital terrestrialradio (also called digital audio broadcasting DAB) and for digitalterrestrial television (also called digital terrestrial videobroadcasting DTVB).

The OFDM transmission method is described in greater detail in theprinted document “Mitteilungen der TU-Braunschweig, Mobilfunktechnik fürMultimedia-Anwendungen” (Information as the Braunschweig technicaluniversity, mobile radio technology for multimedia applications),Professor H. Rohling, volume XXXI, issue 1-1996, in figure 6, page 46.In this method, a serial/parallel conversion is performed for themodulation of, for example, the n subcarriers on the basis of a serialdata stream in the transmitter, a binary code word with word length k(the word length k being dependent on the modulation method used) beingformed in each case for the ith OFDM block in time with block length T′and the jth subcarrier. From the code words formed, the correspondingcomplex modulation symbols, also called transmit symbols in the textwhich follows, are formed with the aid of a transmitter-specificmodulation method, wherein one transmit symbol is allocated to each ofthe k subcarriers at any time i. The spacing of the individualsubcarriers is defined by Δf=1−T′ which guarantees that the individualsubcarrier signals are orthogonal within the useful interval [0, T′]. Bymultiplying the oscillations of the individual subcarriers by thecorresponding modulation symbols or transmit symbols and subsequentlyadding the modulation products formed, the corresponding discrete-timetransmit signal is generated for the ith OFDM block in time. Thistransmit signal is calculated in sampled, i.e. in discrete-time form byan inverse discrete Fourier transform (IDFT) directly from themodulation symbols or transmit symbols of the individual subcarriersconsidered. To minimize intersymbol interferences, each OFDM block ispreceded by a guard interval T_(G) in the time domain which causes anextension of the discrete-time OFDM signal in the interval [−T_(G), 0];compare “Mitteilungen der TU-Braunschweig, Mobilfunktechnik fürMultimedia-Anwendungen”, figure 7. The inserted guard interval T_(G)advantageously corresponds to the maximum delay difference occurringbetween the individual propagation paths occurring during the radiotransmission. By removing the added guard interval T_(G) at the receiverend, a disturbance of the ith OFDM block by, for example, the adjacentOFDM signal in time at time i−1 is avoided, so that the transmit signalis received in interval [0, T′] over all indirect paths and theorthogonality between the subcarriers is retained to its full extent inthe receiver. In the case of a large number of subcarriers, for examplen=256 subcarriers, and correspondingly long symbol periods T=T′+T_(G),the period T_(G) is small compared with T so that the insertion of theguard interval effectively does not significantly impair the bandwidthand only a small overhead is produced. After the transmit signalreceived at the input of the receiver is sampled in the baseband by anA/D converter, and after the useful interval has been extracted, i.e.after the guard interval T_(G) has been eliminated, the receivedtransmit signal is transformed into the frequency domain with the aid ofa discrete Fourier transform (DFT); i.e., the received modulationsymbols or, respectively, the received receive symbols are determined.From the receive symbols determined, the corresponding receive codewords are generated via a suitable demodulation method, and from thesethe received serial data stream is formed by parallel/serial conversion.Avoiding intersymbol interference in OFDM transmission methodsconsiderably reduces the computing effort in the respective receiver asa result of which the OFDM transmission technology is used, for example,for the terrestrial transmission of digital television signals; forexample, the transmission of broadband data streams with a transmissionrate of 34 Mbit/s per radio channel.

To transmit the serial data stream to be transmitted with the aid of theOFDM transmission method, absolute or, respectively, differentialmodulation methods and corresponding coherent or, respectively,incoherent demodulation methods are used. Although the orthogonality ofthe subcarriers is retained in its full extent by using the OFDMtransmission method when transmitting the transmit signal formed via the“radio channel” transmission medium, both the phase and the amplitude ofthe transmitted discrete-frequency and frequency-selective transmitsignals are changed by the transmission characteristics of the radiochannel. The influence of the radio channel on amplitude and phase takesplace subcarrier-specifically on the individual subcarriers which ineach case have a very narrow bandwidth. In addition, noise signals areadditively superimposed on the transmitted useful signal. When coherentdemodulation methods are used, a channel estimation is required whichdepends on considerable technical and economic expenditure for itsimplementation depending on the quality requirements and which alsoreduces the performance of the transmission system. Advantageously,differential modulation methods and corresponding incoherentdemodulation methods are used in which any elaborate radio channelestimation can be dispensed with. In the case of differential modulationmethods, the information to be transmitted is not transmitted directlyby selection of the modulation symbols or the discrete-frequencytransmit symbols but by changing the discrete-frequency transmitsymbols, which are adjacent in time, on the same subcarrier. Examples ofdifferential modulation methods are the 64-level 64-DPSK (differentialphase shift keying) and the 64-DAPSK (differential amplitude and phaseshift keying) methods. In the 64-DAPSK, both the amplitude andsimultaneously the phase are differentially modulated.

In the case of large delay differences between the individual signalpaths, i.e. in the case of strong multipath propagation, differenttransmission-channel-related attenuations may occur between theindividual received subcarriers with attenuation differences of up to 20dB and more. The received subcarriers having high attenuation values or,respectively, the subcarriers having low S/N values (also called thesignal power/noise power ratio) have a very large symbol error rate as aresult of which the total bit error rate rises considerably over allsubcarriers. In the case of subcarriers modulated with the aid ofcoherent modulation methods, it is already known to correct theattenuation losses caused by the frequency-selective transmissioncharacteristics of the transmission medium (also called the transferfunction H(f)) with the aid of the inverse transfer function (alsocalled 1/H(f)) at the receiving end. The frequency-selective attenuationlosses are then determined, for example, by evaluating reference pilottones transmitted and in each case are allocated to certain subcarriers.This method for equalizing the transmission channel at the receivingend, however, causes a great increase in noise in the subcarriers withlow S/N values. The bit error rate caused by the increase in noise insubcarriers with low S/N values cannot even be improved by introducingchannel coding so that the total transmission channel capacity of thefrequency-selective transmission medium, which is possible over allsubcarriers, is not achieved in spite of equalization of thetransmission channel at the receiving end.

In known methods for improving the transmission quality in multicarriersystems as are known, for example, from the document “Comparison betweenadaptive OFDM and single carrier modulation with frequency domainequalization”, A. Czylwik, IEEE Vehicular Technology Conference, USA,New York, vol. Conf. 47, 1997, pp. 865–869, XP000736731, ISBN:0-77803-3660-7, the transfer function of the channel is estimated viainformation already transmitted. It is assumed here that thecharacteristics of the radio channel change only slowly in time. Theestimated transfer function is transmitted back to the transmitter fromthe receiving station via signaling stations.

In a multicarrier method according to U.S. Pat. No. 5,673,290,transmission parameters of a communication line are measured. Themodulation method of each carrier is then adapted to the measuredparameters.

The present invention is thus, directed toward achieving maximumutilization of the available transmission resources of the transmissionmedium during the transmission of information via a transmission mediumhaving frequency-selective transmission characteristics. In particular,it is intended to achieve maximum utilization of the transmissionresources of all multipath components or subcarriers when using amulticarrier method.

SUMMARY OF THE INVENTION

In the method according to the present invention for transmittinginformation via a transmission medium having certain transmissioncharacteristics with the aid of a multicarrier method, the informationto be transmitted is transmitted by a transmit signal having a number offrequency-specific subcarriers to a second unit via the transmissionmedium. An important aspect of the method according to the presentinvention is that frequency-selective transmission characteristics ofthe transmission medium are determined in the first unit and then thefrequency-specific subcarriers of the transmit signal are adapted to thefrequency-selective transmission characteristics of the transmissionmedium which have been determined.

A key advantage of the method according to the present invention is thatdue to the channel equalization at the transmitting end and,respectively, adaptation of the frequency-specific subcarriers of thetransmit signal to be sent out at the transmitting end to thefrequency-selective transmission characteristics of the transmissionmedium which have been determined, all subcarriers of the transmitsignal transmitted via the transmission medium have the same receivelevels or, respectively, signal amplitude values. Thus the same signalpower/noise power ratios S/N at the input of the second unit. Inconsequence, all subcarriers of the transmit signal can be modulatedwith the same number of modulation levels at the transmitting end sothat maximum utilization of the transmission resources of the individualsubcarriers of the transmit signal, and thus maximum utilization of thetransmission resources of the transmission medium is achieved. Due tothe fact that the subcarriers of the transmit signal are modulated withthe same number of modulation levels, the expenditure for controllingthe modulation and demodulation and, especially, the overhead intransmitting the modulation and demodulation control information, forexample via a separate control channel of the transmission medium, isminimized. Advantageously, the frequency-selective channel equalizationaccording to the present invention at the transmitting end prevents theincrease in level of the noise signal duly caused in the case of channelequalization at the receiving end and associated with an increase in biterror probability.

According to an advantageous embodiment of the method according to thepresent invention, the frequency-selective transmission characteristicsof the transmission medium are determined in the second unit andfrequency-specific subcarriers of another transmit signal formed withthe aid of a multicarrier method and transmitted from the second unit tothe first unit are adapted to the frequency-selective transmissioncharacteristics of the transmission medium which have been determined.By determining the frequency-selective transmission characteristics bothin the first unit and in the second unit, the channel equalization ofthe transmit signal at the transmitting end can be advantageouslyimplemented both in the downstream direction and in the upstreamdirection, as a result of which the utilization of the availabletransmission resources of the transmission medium arranged between thefirst unit and the second unit is further improved.

The frequency-selective transmission characteristics are advantageouslydetermined with the aid of the transmit signal transmitted to the firstunit and, respectively, second unit via the transmission medium, inwhich arrangement at least one subcarrier of the transmit signal is usedfor transmitting at least one pilot signal. Due to the transmission andevaluation of pilot signals at the receiving end, detection of thetransmission characteristics of the transmission medium arranged betweenthe first unit and the second unit can be achieved with little technicaland economic expenditure. In particular, the transfer function H(f) ofthe transmission medium and, in particular, the absolute value of thetransfer function |H(f)| can be determined in a particularly simplemanner by evaluating received, frequency-selective pilot signals.

The at least one subcarrier of the transmit signal for transmitting theat least one pilot signal is advantageously modulated by a phasemodulation method, wherein the pilot signal is a certain referenceamplitude. Due to this advantageous embodiment, the subcarriers of thetransmit signal utilized for the transmission of pilot signals areadditionally used, at least partially, for transmission of usefulinformation or, respectively, digital data streams. Thus, a furtherimprovement in the utilization of the transmission resources of thetransmission medium is achieved.

In the case of transmit signals having a large number of subcarriers,the transmission medium has virtually identical transmission parametersfor adjacent subcarriers. According to a further advantageous embodimentof the method according to the present invention, the amplitude-specificand/or phase-specific transmission characteristics of adjacentsubcarriers of the incoming transmit signal are averaged for determiningthe frequency-selective transmission characteristics of the transmissionmedium. Due to the advantageous averaging over the transmissioncharacteristics of a number of subcarriers, arranged adjacently in thefrequency domain, of the transmit signal which have been determined, thenumber of estimated values, thus the accuracy of the channel estimationat the transmitting end, is two-dimensionally increased without thespectral distance to adjacent subcarriers becoming too large.

In the case of fast time variations of the transmission media exhibitingtransmission characteristics or, respectively, in the case oftime-variant transmission media, time-selective, amplitude-specificand/or time-selective, phase-specific transmission characteristics ofthe transmission medium are determined according to a furtheradvantageous embodiment of the method according to the presentinvention. Pursuant to such method, a number of frequency-selective,amplitude-specific and/or frequency-selective, phase-specifictransmission characteristics, which are determined over a period oftime, are stored in the respective unit and then the average over thestored frequency-selective, amplitude-specific and/orfrequency-selective, phase-specific transmission characteristics isformed. Following this, the frequency-specific subcarriers of thetransmit signal are adapted to the transmission characteristics of thetransmission medium which are averaged over time. Due to the averagingover a number of frequency-selective transmission characteristics of thetransmission medium which have been determined successively in time, thefirst derivation of the time variations of the transmissioncharacteristics of the transmission medium is corrected during thedetection of the transmission characteristics, which further improvesthe quality of the channel estimation at the transmitting end and thechannel equalization at the transmitting end.

The frequency-selective transmission characteristics which have beendetermined are advantageously transmitted by the first unit to thesecond unit and the frequency-specific subcarriers of the furthertransmit signal are adapted to the transmitted transmissioncharacteristics of the transmission medium in the second unit. Due tothis advantageous variant of this embodiment, the transmissioncharacteristics of the transmission medium arranged between the firstunit and the second unit are only determined in one unit and the resultof the determination is transmitted in parameterized form to the secondunit as a result of which the expenditure for implementing the channelequalization at the transmitting end is kept low both in the first unitand in the second unit.

According to a further advantageous embodiment of the present invention,the signal power/noise power ratio S/N is determined for each subcarrierof the transmit signal in the determination of the frequency-selectivetransmission characteristics and the subcarriers are used fortransmitting information (dsu, dsd) in dependence on the signalpower/noise power ratio S/N determined in each case. In the case of asignal power/noise power ratio S/N measured below a limit value, thecorresponding subcarrier is advantageously not used for transmittinginformation. Due to the deactivation of the subcarriers having, in eachcase, an inadequate signal power/noise power ratio S/N and thus notbeing usable for information transmission, the transmitting power of theremaining subcarriers used for information transmission can becorrespondingly increased. Increasing the transmitting power of thesubcarriers used for information transmission further reduces their biterror probability.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Preferred Embodiments and the Drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a centralized transceiver unit implementing an OFDMtransmission method; and

FIG. 2 shows a decentralized transceiver unit which is connected to acentralized transceiver unit according to FIG. 1 via the transmissionmedium “radio channel” and implements an OFDM transmission method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 in this case show a first and second transceiver unitSEE1,2 which can be, for example, modular components of transmitting andreceiving systems implementing wireless communication networks. In thepresent exemplary embodiment, the first transceiver unit SEE1 shown inFIG. 1 is arranged in a base station BS representing the center of aradio cell or of a radio area (not shown) and the second transceiverunit SSE2 shown in FIG. 2 is arranged in a decentralized wirelessnetwork terminating unit RNT representing a wireless subscriber lineunit. FIG. 2 only shows a wireless network terminating unit RNT asrepresentative of decentralized network terminating units allocated tothe base station BS or, respectively, the radio cell. To eachdecentralized wireless network terminating unit RNT, at least onedecentralized communication terminal (not shown) an be connected whichcan be constructed, for example, as multimedia communication terminal oras ISDN-oriented telephone terminal. The decentralized wireless networkterminating units RNT and the decentralized communication terminalsconnected to them can be connected to a higher-level communicationnetwork connected to the base station BS for example an ISDN-orientedlandline network or a broadband-oriented multimedia communicationnetwork (not shown), via the wireless transmission medium “radiochannel”.

The first transceiver unit SEE1 shown in FIG. 1 has a data input ED towhich a digital serial data stream dsd to be transmitted from thehigher-level communication network to the decentralized wireless networkterminating units RNT is conducted. The data input ED is connected to aninput EO of an OFDM transmit unit SOB which is arranged in the firsttransceiver unit SEE1 and in which a method, already explained in theintroduction to the description, for forming an OFDM signal sd having nsubcarriers is implemented. The OFDM transmit unit SOB exhibits amodulator MOD which modulates the n subcarriers of the OFDM signal sdand which is connected via n outputs AM1 . . . n and n link lines to nfrequency-selective inputs EF1 . . . n, associated with the nsubcarriers of the OFDM signal sd, of a transformation unit IFFT forperforming a discrete inverse fast Fourier transformation. Thetransformation unit IFFT is used for generating from thesubcarrier-specific modulation symbols or, respectively, transmitsymbols SS1 . . . n conducted from a modulator MOD to thefrequency-selective inputs EF1 . . . n of the transformation unit IFFT adiscrete-time OFDM signal. In the OFDM transmit unit SOB, other units(not shown) such as parallel/serial converters, digital/analogconverters, filter units, and amplitude limiters, for converting thediscrete-time OFDM signal into the analog OFDM signal sd, for example byadhering to spectrum masks defined for wireless communication networksor mobile radio systems and stipulated by ETSI standardization, arearranged. The OFDM transmit unit SOB is connected via an output AO to aninput EH of a radio-frequency transmit unit HS which is connected via anoutput AH and via an antenna output AS of the first transceiver unitSEE1 to a transmit antenna SA arranged in the external area of the basestation BS. The analog OFDM transmit signal sd is amplified by atransmit amplifier, not shown, arranged in the radio-frequency transmitunit HS, is mixed into the radio-frequency or RF band and subsequentlytransmitted via the transmit antenna SA and via the wirelesstransmission medium “radio channel” to the decentralized networkterminating units RNT arranged in the radio cell of the base station BS(also called the downstream direction).

Furthermore, an OFDM receiving unit EOB is arranged in the firsttransceiver unit SEE1, which is connected via an input EO to an outputAH of a radio-frequency receiving unit HE. The radio-frequency receivingunit HE has an input EH which is connected to a receiving antenna EAarranged in the external area of the base station BS, via an antennainput ES of the first transceiver unit SEE 1. An OFDM signal sutransmitted by a decentralized network terminating unit RNT to the basestation BS and received at the receiving antenna EA of the base stationBS is down converted to the intermediate-frequency band or,respectively, the baseband by a conversion device (not shown) arrangedin the radio-frequency receiving unit HE and then forwarded to the inputEO of the OFDM receiving unit EOB.

In the OFDM receiving unit EOB, a transformation unit FFT forimplementing a discrete fast Fourier transform and having a number offrequency-selective outputs AF1 . . . n is arranged, eachfrequency-selective output AF1 . . . n being associated with onesubcarrier of the received OFDM signal. After previous discretizationand digitization with the aid of an analog/digital converter (notshown), the OFDM signal su received and down converted into theintermediate-frequency band or baseband, respectively, is transformedinto the frequency domain with the aid of the fast Fourier transformimplemented by the transformation unit FFT; i.e., the modulation symbolsor receive symbols es1 . . . n of the respective subcarriers containedin the OFDM signal are determined and then forwarded to thecorresponding frequency-selective outputs AF1 . . . n of thetransformation unit FFT. The outputs AF1 . . . n of the transformationunit FFT are connected to n inputs EM1 . . . n of a demodulator DMOD vian link lines. From the receive symbols es1 . . . n forwarded to thedemodulator DMOD from the transformation unit FFT, the correspondingreceive code words transmitted via the respective subcarriers aredetermined with the aid of a demodulation method implemented in thedemodulator DMOD. The receive code words which have been determined arethen converted with the aid of a parallel/serial converter (not shown),associated with the OFDM receiving unit EOB, into a serial digital datastream deu which is forwarded, for example, to the higher-levelcommunication network via a data output AD of the first transceiver unitSEE1.

The second transceiver unit SEE2, arranged in the decentralized wirelessnetwork terminating unit RNT according to FIG. 2, has an OFDM receivingunit EON which is connected via an input EO to an output AH of aradio-frequency receiving unit HE arranged in the second transceiverunit SEE2. The radio-frequency receiving unit HE is connected via aninput EH to a receiving antenna EA arranged in the external area of thenetwork terminating unit RNT. The OFDM signal sd transmitted by the basestation BS to the network terminating unit RNT and received at thereceiving antenna EA is down converted into the intermediate-frequencyband or, respectively, the baseband by a conversion device (not shown),arranged in the radio-frequency receiving unit HE, and then forwarded tothe input EO of the OFDM receiving unit EON. In the OFDM receiving unitEON, a transformation unit FFT for implementing a discrete fast Fouriertransform and exhibiting a number of frequency-selective outputs AF1 . .. n is arranged, each frequency-selective output AF1 . . . n beingassociated with one subcarrier of the received OFDM signal sd. Using thefast Fourier transform implemented by the transformation unit FFT, theOFDM signal sd received and down converted into theintermediate-frequency band or baseband, respectively, is transformedinto the frequency domain after previous discretization and digitizationwith the aid of an analog/digital converter (not shown); i.e., themodulation symbols or receive symbols es1 . . . n of the respectivesubcarriers contained in the received OFDM signal sd are determined andthen forwarded to the corresponding frequency-selective outputs AF1 . .. n of the transformation unit FFT. The n outputs AF1 . . . n of thetransformation unit FFT are connected via n link lines to n inputs EK1 .. . n of a channel estimation unit KS which is connected tocorresponding frequency-selective inputs EM1 . . . n of a demodulatorDMOD arranged in the OFDM receiving unit EON via n outputs AK1 . . . nand n link lines. The frequency-selective receive symbols es1 . . . ntransmitted by the transformation unit FFT to the channel estimationunit KS are forwarded to the inputs EM1 . . . n of the demodulator DMOD.In the channel estimation unit KS, a first evaluating device UF isarranged via which the frequency-selective amplitude-specifictransmission channel characteristics of the transmission medium “radiochannel” are determined from the receive symbols es1 . . . n conductedto the channel estimation unit KS; i.e., the frequency-selectiveamplitude distortions (also called amplitude response or absolute valueof the transfer function of the radio channel |H(f)|) caused by thetransmission medium “radio channel” are determined for each subcarrier.Furthermore, the S/N ratio is determined for each subcarrier from theincoming receive symbols es1 . . . n via a further evaluating device SNarranged in the channel estimation unit KS. From the frequency-selectiveamplitude response |H (f)| determined and the frequency-selective S/Nratio determined, an information signal is transmitting the results ofthe determination is generated by a signal generating device (notshown), arranged in the channel estimation unit KS, which informationsignal is forwarded to a control output SA of the OFDM receiving unitEON via an output ASK of the channel estimation unit KS.

The frequency-selective receive symbols es1 . . . n forwarded to thedemodulator DMOD from the channel estimation unit KS are converted intothe receive code words transmitted via the respective subcarriers by ademodulation method implemented in the demodulator DMOD. From thereceive code words determined, a serial/digital data stream ded is thenformed with the aid of a parallel/serial converter (not shown), which isassociated with the OFDM receiving unit EON, which data stream isconducted to a data output AS of the second transceiver unit SEE2 via anoutput AO of the OFDM receiving unit EON and is then transmitted, forexample, to a decentralized destination communication terminal, notshown, which is connected to the decentralized network terminating unitRNT.

The control output SA of the second transceiver unit SEE2 arranged inthe decentralized network terminating unit RNT is connected via a linkline VL to a control input SE of an OFDM transmit unit SON arranged inthe second transceiver unit SEE2, in which transmit unit a method forforming an OFDM signal su to be transmitted in the upstream directionand having n subcarriers is implemented. The OFDM transmit unit SON isconnected via an input EO to a data input ES of the second transceiverunit SEE2 to which, for example, a digital serial data stream dsu to betransmitted from a decentralized communication terminal via the wirelesstransmission medium “radio channel” to the higher-level communicationnetwork is conducted. The digital serial data stream dsu is divided inton parallel sub-data streams, or converted in parallel form,respectively, by a serial/parallel converter (not shown), which isassociated with the OFDM transmit unit SON, each of the n sub-datastreams being allocated to one of the n subcarriers of the OFDM signal.The n parallel sub-data streams are conducted to a modulator MODarranged in the OFDM transmit unit SON and modulating the n subcarriersof the OFDM signal os, the incoming n sub-data streams being convertedinto n frequency-selective modulation symbols or transmit symbols ss1 .. . n associated with the n subcarriers of the OFDM signal by amodulation method implemented in the modulator MOD. The nfrequency-selective transmit symbols ss1 . . . n formed are forwarded ton outputs AK1 . . . n of the modulator MOD which is connected to nfrequency-selective inputs EE1 . . . n of a channel equalizer unit EZ,which are associated with the n subcarriers of the OFDM signal su. Thechannel equalizer unit EZ has a control input ESS which is connected tothe control input SE of the OFDM transmit unit SON and is thus connectedto the output ASK of the channel estimation unit KS arranged in the OFDMreceiving unit EON via the link line VL.

The channel equalizer unit EZ has capabilities for adapting the transmitsymbols ss1 . . . n formed by the modulator MOD and forwarded to thechannel equalizer unit EZ to the frequency-selective amplitude-specifictransmission channel characteristics of the transmission medium “radiochannel” determined in the OFDM receiving unit EON (also called“equalization of the amplitude response” or “amplitude equalization”);i.e., the amplitudes of the frequency-selective transmit symbols ss1 . .. n are corrected in dependence on the information signal transmitted tothe control input ESS. For example, the frequency-selective transmitsymbols ss1 . . . n are multiplied by the inverse of the absolute valueof the transfer function of the radio channel determined, in this case1/|H(f)|. The n corrected frequency-selective transmit symbols ss′1 . .. n are forward to n outputs AZ1 . . . n of the channel equalizer unitEZ which are connected to corresponding n frequency-selective inputs EF1. . . n, allocated to the n subcarriers of the OFDM signal, of atransformation unit IFFT for performing a discrete inverse fast Fouriertransformation. Using the transformation unit IFFT, a discrete-time OFDMsignal is calculated from the subcarrier-specific and corrected transmitsymbols ss′1 . . . n forwarded from the channel equalizer unit EZ to thefrequency-selective inputs EF1 . . . n of the transformation unit IFFT.In the OFDM transmit unit SON, further units (not shown), such asparallel/serial converters, digital/analog converters, filter units, andamplitude limiters, for converting the discrete-time OFDM signal into ananalog OFDM transmit signal su, for example by adhering to theaforementioned ETSI spectrum masks, are arranged. The OFDM transmit unitSON is connected via an output AO to an input EH of a radio-frequencytransmit unit HS which is connected to a transmit antenna SA arranged inthe external area of the decentralized network terminating unit RNT viaan output AH and via an antenna output AS of the second transceiver unitSEE2. The analog OFDM transmit signal su is amplified by a transmitamplifier, not shown, which is arranged in the radio-frequency transmitunit HF, is converted into the radio-frequency band or RF band and thentransmitted to the base station BS via the transmit antenna SA and viathe wireless transmission medium “radio channel” in the upstreamdirection.

It should be noted that the exemplary embodiment described onlyrepresents a functional description of the method according to thepresent invention; i.e., the embodiment of the first and secondtransceiver unit SEE1,2 described in the exemplary embodiment can alsobe implemented by alternative variants of the embodiment. For example,the radio-frequency transmitting unit and receiving unit HS, HE arrangedin each case in a transceiver unit SEE1,2 can be replaced by aradio-frequency converter unit (not shown), where the respectivetransmitting and receiving paths are separated via a switch, not shown.

In the text which follows, the method according to the present inventionfor maximum utilization of the transmission resources provided by thewireless transmission medium “radio channel” is explained in greaterdetail.

The radio-frequency transmitting and receiving units HS, HE arranged inthe first and second transceiver unit SEE1,2 are designed in such amanner that OFDM signals sd, su transmitted in the downstream andupstream direction are transmitted in the TDD (time division duplex)transmission method. In the TDD transmission method, the information tobe transmitted between the base station BS and the wirelessdecentralized network terminating units RNT are alternately transmittedwith the aid of signal bursts of a particular extent in time which aresent out in the same frequency range. In this method, the transceiverunits SEE1,2 arranged in the network terminating units RNT and in thebase station BS are alternately switched to transmit and receive mode.When the TDD transmission method is used, the wireless transmissionmedium “radio channel” exhibits reciprocal characteristics. That is, theOFDM signal sd sent out in bursts in the downstream direction by thebase station BS and received by a decentralized network terminating unitRNT, it is possible to determine or, respectively, estimate thefrequency-selective amplitude-specific and/or phase-specifictransmission channel characteristics of the transmission medium “radiochannel” for the OFDM signal su to be transmitted in the upstreamdirection by the decentralized network terminating unit RNT.

According to a first variant of the embodiment of the method accordingto the present invention, a differential phase modulation method(differential phase shift keying, for example a 64 DPSK), is implementedin the modulator MOD arranged in the OFDM transmit unit SOB of the firsttransceiver unit SEE1. When a differential modulation method is used, nocarrier recovery of the received OFDM signal sd and no precise recoveryof the bit clock is required in the subsequent demodulation in thecorresponding OFDM receiving unit EON or, respectively, the demodulatorDMOD arranged therein. To provide for a determination of thefrequency-selective transmission characteristics of the transmissionmedium “radio channel”, also called channel estimation in the text whichfollows, at the receiving end, the modulator MOD arranged in the basestation BS is designed in such a manner that a particular number of thetransmit symbols ss1 . . . n present at the n outputs AM1 . . . n of themodulator MOD are designed as pilot symbols with defined referenceamplitude; i.e., some of the subcarriers of the OFDM signal sd to betransmitted in the downstream direction are used for transmitting ineach case a pilot tone or pilot signal having a defined referenceamplitude. For example, 10% of the subcarriers of the OFDM signal sd,which can be used for information transmission, are used fortransmitting pilot tones.

From the OFDM signal received at the receiving antenna EA of the networkterminating unit RNT, the transmitted receive symbols es1 . . . n of therespective subcarriers of the received OFDM signal sd are determined bythe transformation unit FFT arranged in the OFDM receiving unit EON andforwarded to the channel estimation unit KS.

From the receive symbols es1 . . . n present at the inputs EK1 . . . nand designed as pilot symbols, the frequency-selective,amplitude-specific transmission characteristics or frequency-selectiveattenuation characteristics of the transmission medium “radio channel”FK arranged between the base station BS and the decentralized wirelessnetwork terminating unit RNT are determined. That is, the amplituderesponse or absolute value of the transmission function |H(f)| of thetransmission medium “radio channel” FK is determined by the firstevaluating device HF arranged in the channel estimation unit KS. Withthe aid of the information signal is, the transmission characteristicsof the transmission medium “radio channel” FK which have been determinedare then transmitted to the control input SE of the OFDM transmittingunit SON arranged in the decentralized network terminating unit RNT viathe link line VL. Furthermore, the receive symbols es1 . . . n forwardedfrom the channel estimation unit KS to the n inputs EM1 . . . n of thedemodulator DMOD are converted in the OFDM receiving unit EON, with theaid of the differential or, respectively, incoherent demodulation methodimplemented in the demodulator DMOD, into the receive code wordstransmitted via the respective subcarriers of the OFDM signal sd, fromwhich code words the serial digital data stream ded conducted to theoutput AS of the second transceiver unit SEE2 is formed.

According to the present invention, the OFDM signal to be transmitted tothe base station BS in the upstream direction is generated depending onthe transmission channel characteristics of the transmission medium“radio channel” determined by the OFDM receiving unit EON and forwardedto the OFDM transmitting unit SON. For this purpose, the digital serialdata stream dsu received at the input EO of the OFDM transmitting unitSON arranged in the second transceiver unit SEE2 and to be transmittedto the base station BS is converted into parallel form and convertedinto the transmit symbols ss1 . . . n associated with the n subcarriersof the OFDM signal, with the aid of the modulator MOD. The transmitsymbols ss1 . . . n formed are forwarded to the n inputs EE1 . . . n ofthe channel equalizer unit EZ and adapted to the frequency-selectiveamplitude-specific transmission channel characteristics of thetransmission medium “radio channel” FK, which have been determined bythe correction device 1/HF arranged in the equalizer unit (also calledamplitude equalization at the transmitting end). The amplitudeequalization at the transmitting end implemented by the correctiondevice 1/HF takes place in such a manner that the transmit symbols ss1 .. . n of the individual subcarriers of the OFDM signal su are multipliedby a factor representing the absolute value of the inverse of thetransfer function H_(n)(f) determined. In this case, 1/|H_(n)(f)| for0≦n≦N−1, wherein n represents the length of the Fourier transformimplemented in the transformation unit IFFT and H_(n)(f) represents thetransfer function of the nth subcarrier of the OFDM signal.

The frequency-selective amplitude equalization according to the presentinvention, at the transmitting end, which has been described, has theeffect that all subcarriers of the OFDM signal su transmitted to thebase station BS from the decentralized network terminating unit RNT inthe upstream direction have the same receive levels or signal amplitudevalues when they arrive at the receiving antenna EA of the base stationBS. Since all subcarriers of the OFDM signal su received in the basestation BS have the same receive level, the signal power/noise powerratio S/N is identical for all subcarriers. Thus, all subcarriers can bemodulated with the same number of modulation levels at the transmittingend; i.e., with the aid of the OFDM transmitting unit SON arranged inthe decentralized network terminating unit RNT or, respectively, withthe aid of the modulator MOD arranged there. This achieves maximumutilization of the transmission resources of the individual subcarriersof the OFDM signal su. For example, if the decentralized networkterminating units RNT are arranged close to the base station BS, theindividual subcarriers of the OFDM signal su to be transmitted to thebase station BS in the upstream direction can be modulated with the aidof the 64-QAM (quadrature amplitude modulation). As the distance betweenthe decentralized network terminating unit RNT and the base station BSincreases, i.e. with increasing attenuation characteristics of thetransmission medium “radio channel” FK, the number of modulation levelsis reduced. Due to the identical S/N ratio of the subcarrier of the OFDMsignal su received in the base station BS, no subcarrier-individualnumber of modulation levels is required for controlling the demodulationof the received OFDM signal so that the control effort for modulatingand demodulating the OFDM signal su is advantageously minimum. Byavoiding the requirement of subcarrier-individual number of modulationlevels, no additional overhead is generated for transmitting additionalcontrol information controlling the subcarrier-individual modulation anddemodulation, thus preventing the transmission capacity of thetransmission medium “radio channel” from being reduced.

As an alternative, the transmission power of the OFDM signal su to besent out can be correspondingly reduced instead of increasing the numberof modulation levels of the OFDM signal su to be sent in the upstreamdirection. The transmission power can be lowered, for example, in theradio-frequency transmit unit HS of the decentralized networkterminating unit RNT. Lowering the transmission power minimizes themutual interference of the subcarriers of OFDM signals sd, su sentwithin a radio cell, also called intercell interference (ICI) and, as aresult, the transmission capacity of the total system arranged within aradio cell is increased.

According to a further advantageous variant of the embodiment of themethod according to the present invention, the channel estimation unitKS of the OFDM receiving unit EON arranged in the decentralized networkterminating unit RNT has a further evaluating device SN for detectingthe subcarrier-individual S/N ratios of the respective subcarriers ofthe received OFDM signal sd. The subcarrier-individual S/N ratiosdetected in each case with the aid of the further evaluating device S/Nare additionally transmitted, in addition to the detectedamplitude-specific transmission characteristics H(f) of the transmissionmedium FK with the aid of the information signal is via the link line VLto the OFDM transmitting unit SON arranged in the decentralized networkterminating unit RNT or, respectively, to the channel equalizer unit EZarranged there.

In the channel equalizer unit EZ a further correction device, (1/HF2),is arranged via which the subcarriers having unfavorable S/N ratios orthe subcarriers having an S/N ratio which is measured below a limitvalue, are deactivated in dependence on the S/N ratios transmitted tothe control input ESS, and thus are not used for informationtransmission. For example, in the case of decentralized networkterminating units RNT which are at a large distance from the basestation BS, only every second or fourth subcarrier of the OFDM signal suto be sent to the base station BS is used for information transmission,the transmission power of the subcarriers used for informationtransmission being correspondingly increased. Increasing thetransmission power of the subcarriers used for information transmissionfurther reduces the bit error probability. Deactivated subcarriers ofthe received OFDM signal can be detected by simple amplitude calculationin the OFDM receiving unit EON, EOB.

Since the determination of the frequency-selective amplitude-specifictransmission characteristics of the transmission medium “radio channel”FK, implemented in the decentralized network terminating unit RNT at thetransmitting end, only requires the evaluation of the amplitude value ofthe pilot symbols or pilot tones transmitted from the base station BS tothe decentralized network terminating unit RNT by the channel estimationunit KS arranged in the decentralized network terminating unit RNT, thephase information of the pilot symbols or pilot tones of the OFDMsignals sd, sent from the base station BS to the decentralized networkterminating unit RNT, can be additionally used for transmitting thedigital information dsd. The subcarriers of the OFDM signal sd whichtransmits pilot symbols or pilot tones can be modulated, for example,with the aid of an absolute or differential phase modulation method withdefined reference amplitude as a result of which an advantageousutilization of the transmission capacity of the transmission medium“radio channel” is achieved.

The OFDM transmission units SOB, SON arranged in the base station BS ordecentralized network terminating unit RNT or, respectively, themodulators MOD arranged there, are designed in such a manner that thesubcarriers of the OFDM signals sd, su which are not used for thetransmission of pilot tones are modulated with a coherent or absolutemodulation method, for example an m-level QAM, since m-level QAMmodulation methods can also be used in transmission media withunfavorable S/N ratios.

When coherent m-level modulation methods are used, additional methods(not shown) for the channel estimation or channel equalization at thereceiving end, required according to the prior art, especially for phaseequalization of the subcarriers received in each case of the receivedOFDM signal sd, su are required in the corresponding OFDM receivingunits EON, EOB arranged in the base station BS and, respectively, thedecentralized network terminating unit RNT. To provide for correction ofthe phase angles of the incoming subcarriers in the OFDM receiving unitEON, EOB, the first subcarrier of the OFDM signal sd, su is transmittedwith a defined phase, e.g. φ=0 degrees, by the OFDM transmitting unitSOB, SON. The phase of the first subcarrier is rotated by, for example,Δφ by the transmission medium “radio channel” FK. The second subcarrierarranged closely adjacently to the first subcarrier is also rotated byΔφ in this process. To restore the original phase angles of thetransmitted OFDM signal sd, su, the second subcarrier must be multipliedby the complex factor e^(−jΔφ) by the correction device arranged in theOFDM receiving unit EON, EOB. The phase shift Δφ of the first subcarriercaused by the transmission medium “radio channel” FK can be detected,and the phase angle of the adjacent second subcarrier of the receivedOFDM signal can be correspondingly corrected with the aid of thecorrection device due to the pilot tone with defined transmitting phasetransmitted via the first subcarrier. After the correction of the phaseangle or phase equalization at the receiving end, the informationtransmitted via the second subcarrier is decided with the aid of thedemodulator. The phase shift of the second subcarrier is determined independence on the result of the decision. The phase angle of the thirdsubcarrier is then corrected in the manner described via the phase shiftof the second subcarrier determined, etc.

According to a further advantageous embodiment of the method accordingto the present invention, the OFDM receiving unit EOB arranged in thebase station BS also has a channel estimation unit KS, not shown, viawhich the receive symbols es1 . . . n transmitted with the aid of thereceived OFDM signal su are evaluated and, from this, thefrequency-selective, amplitude-specific radio channel characteristics ofthe transmission medium “radio channel” FK are evaluated in the mannerdescribed and are transmitted via a link line (not shown), to a furtherchannel equalizer unit (not shown), which is arranged in the OFDMtransmitting unit SOB of the base station BS. Due to this advantageousembodiment, the OFDM signals sd to be transmitted to the decentralizednetwork terminating units RNT from the base station BS in the downstreamdirection, and the subcarriers contained therein, can also be adapted tothe transmission characteristics of the transmission medium “radiochannel”. The equalization of the amplitude response at the transmittingend achieved in this manner, both in the downstream direction and in theupstream direction, further improves the utilization of the transmissioncapacity of the transmission medium “radio channel” FK. However, thispresupposes that some of the subcarriers of the OFDM signal su to betransmitted from the decentralized network terminating unit RNT to thebase station BS are used for transmitting pilot symbols or pilot tonesas already described. The modulator MOD arranged in the OFDMtransmitting unit SON of the decentralized network terminating unit RNTis advantageously designed in such a manner that the subcarriers of theOFDM signal su which are used for the transmission of pilot symbols aremodulated with the aid of a phase modulation method, for example a QPSKmodulation methods with defined reference transmit amplitude. By usingphase modulation, the pilot symbols or pilot tones transmitted in theupstream direction are also at least partially used for transmitting thedigital data stream dsu.

To increase the accuracy of channel estimation in the decentralizednetwork terminating unit RNT and possibly in the base station BS, thesubcarriers of an OFDM signal sd, su which, in each case, transmit pilottones or pilot symbols can be transmitted with increased power.

According to a further variant of the embodiment, the channel estimationat the transmitting end is only performed by the channel estimation unitKS arranged in the decentralized network terminating unit RNT and thenthe frequency-selective, amplitude-specific transmission characteristicsof the transmission medium “radio channel” which have been determinedare transmitted in parameterized form to the base station BS or,respectively, the OFDM transmitting unit SOB arranged there. Theequalization of the amplitude response of the subcarriers of the OFDMsignal sd to be transmitted in the downstream direction from the basestation BS is carried out by a channel equalizer unit (not shown), whichis arranged in the OFDM transmitting unit SOB of the base station BS,with the aid of the parameterized transmission characteristicstransmitted.

Advantageously, only the changes of the transmission characteristicswith time are transmitted to the base station BS and thus the overheadduring the transmission of the transmission characteristics isminimized.

In the case of OFDM signals sd, su, having a large number ofsubcarriers, the transmission medium “radio channel” FK has virtuallyidentical transmission characteristics for adjacent subcarriers.Advantageously, in addition to the directly adjacent subcarriers, theadjoining subcarriers in the frequency range are also taken intoconsideration for the determination of the frequency-selectivetransmission characteristics of the transmission medium or the channelestimation at the transmitting end performed in an OFDM receiving unitEON, EOB; i.e., an average is formed over determined transmissioncharacteristics of a number of subcarriers arranged adjacently in thefrequency range. The averaging has the advantage that the number ofestimated values, and thus the accuracy of the channel estimation at thetransmitting end is two-dimensionally increased without the spectraldistance from adjacent subcarriers becoming too great.

According to a further advantageous embodiment of the method accordingto the present invention, in the case of radio channels exhibiting afast change with time (also called time-variant transmission channels orradio channels), the OFDM signals following in time; i.e., thosereceived at the receiving antenna EA within a certain period of time or,respectively, the receive symbols es1 . . . n contained therein, arealso taken into consideration in the channel estimation implemented inthe channel estimation unit KS. Implementation of this variant of theembodiment requires the storing of the receive symbols es1 . . . nreceived successively in time or storing of the frequency-selectivetransmission characteristics determined in each case, in a memory, notshown, which is arranged in the first or second transceiver unit SEE1,2. Averaging over a number of receive symbols es1 . . . n in each casebelonging to a subcarrier and received successively in time, within thechannel estimation at the transmitting end performed in the channelestimation unit KS corrects the first generation of the changes withtime of the transmission characteristics of the transmission medium“radio channel” FK during the detection of the transmissioncharacteristics. Advantageously, the subcarriers arranged symmetricallyabout the current subcarrier in the frequency domain or, respectively,the receive symbols es1 . . . n transmitted via this subcarrier, aretaken into consideration during the averaging. As an alternative, theaveraging can also be done in the channel equalizer unit EZ of the OFDMtransmitting unit SON.

The determination of the frequency-selective, amplitude-specifictransmission characteristics of the transmission medium “radio channel”FK, performed with the aid of the evaluating device H(f) in the channelestimation unit KS, also called calculation of the estimated amplitudevalues, is relatively complex. The amplitude values of all receivedreceive symbols es1 . . . n of an OFDM signal sd are calculatedfollowing the calculation rule√{square root over (I ² +Q ²)}=Amplitudewhere I is the imaginary part and Q is the real part of a receivedcomplex receive signal es1 . . . n. The respective frequency-selectiveestimated amplitude values can be calculated serially, at leastpartially, so that the technical complexity or hardware complexity forcalculating the estimated amplitude values is kept low.

According to an advantageous embodiment, the estimated amplitude valuesare calculated from the frequency-selective receive symbols es1 . . . nreceived in each case, with the aid of values stored in a table calledlook-up table. For this purpose, the receive values of the imaginarypart I and of the real part Q of a receive symbol es1 . . . n, which arein each case possible, are combined to form a table address and arestored in the look-up table. Furthermore, to each stored table address,the associated correction factor, 1/|H_(n)(f)| in this cases isallocated and stored in the corresponding table entry. The correctionfactors allocated to the respective table addresses represent the valuesby which the respective transmit symbols ss1 . . . n of the OFDM signalsd, su to be sent out are multiplied. The extent or, respectively,number of entries of the look-up table is advantageously kept small ifit is restricted to one quadrant of the complex plane, transmit symbolsss1 . . . n having negative imaginary and real part values beinginverted before the amplitude equalization at the transmitting end.

According to a further advantageous embodiment, the multiplication ofthe subcarriers or, respectively, of the transmit symbols ss1 . . . n tobe transmitted via the subcarriers, by the correction factor determined,1/|H_(n)(f)| in this case, is implemented by an addition or,respectively, subtraction with values also stored in a look-up table.This advantageous embodiment further reduces the computing effort forcorrecting the transmit symbols during the amplitude equalization.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the invention as set forth in the hereafter appended claims.

1. A method for transmitting information via a transmission mediumhaving particular transmission characteristics, with the aid of amulticarrier method, from a first unit to a second unit, the methodcomprising the steps of: using a first transmit signal to transmit theinformation, the first transmit signal exhibiting a plurality offrequency-specific subcarriers; determining, in the first unit,frequency-selective transmission characteristics of the transmissionmedium using a second transmission signal sent out by the second unit,the second transmission signal exhibiting at least onefrequency-specific subcarrier; adapting, in the first unit, theplurality of frequency-specific subcarriers of the first transmit signalto the frequency-selective transmission characteristics of thetransmission medium which have been determined; determining at least oneof frequency-selective amplitude-specific transmission characteristicsand frequency-selective phase-specific transmission characteristics ofthe transmission medium as the transmission characteristics; andaveraging at least one of the frequency-selective amplitude-specifictransmission characteristics and the frequency-selective phase-specifictransmission characteristics of adjacent subcarriers of one of the firstand second transmit signals for determining the frequency-selectivetransmission characteristics of the transmission medium.
 2. A method fortransmitting information as claimed in claim 1, the method furthercomprising the steps of: determining, in the second unit, thefrequency-selective transmission characteristics of the transmissionmedium; adapting, in the second unit, to the frequency-selectivetransmission characteristics of the transmission medium which have beendetermined, a plurality of frequency-specific subcarriers of the secondtransmit signal formed with the aid of a multicarrier method andtransmitted from the second unit to the first unit.
 3. A method fortransmitting information as claimed in claim 1, the method furthercomprising the step of: determining a transfer function of thetransmission medium during the step of determining thefrequency-selective transmission characteristics of the transmissionmedium.
 4. A method for transmitting information as claimed in claim 3,the method further comprising the step of: representing thefrequency-selective amplitude-specific transmission characteristics ofthe transmission medium by an absolute value of the transfer functionwhich has been determined.
 5. A method for transmitting information asclaimed in claim 1, the method further comprising the steps of:determining the frequency-selective transmission characteristics usingboth the first and second transmit signals; and utilizing at least onesubcarrier of one of the first and second transmit signals fortransmitting at least one pilot signal.
 6. A method for transmittinginformation as claimed in claim 5, the method further comprising thestep of: modulating the at least one subcarrier using a phase modulationmethod for transmitting the at least one pilot signal, wherein the pilotsignal exhibits a particular reference amplitude.
 7. A method fortransmitting information as claimed in claim 1, the method furthercomprising the steps of: determining at least one of time-selectiveamplitude-specific transmission characteristics and time-selectivephase-specific transmission characteristics of the transmission medium;storing a plurality of the frequency-selective amplitude-specifictransmission characteristics and frequency-selective phase-specifictransmission characteristics, determined over a period of time, in therespective one of the first and second units; forming, in each case, anaverage value of the at least one of the stored frequency-selectiveamplitude-specific transmission characteristics and thefrequency-selective phase-specific transmission characteristics; andadapting the subcarriers of the respective one of the first and secondtransmit signals to be transmitted to the transmission characteristicsof the transmission medium which are averaged over time.
 8. A method fortransmitting information as claimed in claim 1, the method furthercomprising the steps of: transmitting the determined frequency-selectivetransmission characteristics from the first unit to the second unit; andadapting the frequency-specific subcarriers of the second transmitsignal to the transmission characteristics of the transmission medium inthe second unit.
 9. A method for transmitting information as claimed inclaim 8, wherein only changes with time of the transmissioncharacteristics are transmitted by the first unit to the second.
 10. Amethod for transmitting information as claimed in claim 4, the methodfurther comprising the step of: multiplying subcarriers of the first andsecond transmit signals by one of an inverse of the determined transferfunction and an inverse of the absolute value of: the determinedtransfer function in the adaptation of the first and second transmitsignals to the transmission characteristics of the transmission medium.11. A method for transmitting information as claimed in claim 1, whereinthe first and second transmit signals transmitted between the first andsecond units are transmitted in a time division duplex transmissionmethod.
 12. A method for transmitting information as claimed in claim 2,the method further comprising the steps of: determining, in thedetermination of the frequency-selective transmission characteristics, asignal power/noise power ratio for each subcarrier of each of the firstand second transmit signals; and utilizing the respective subcarrier ofeach of the first and second transmit signals for the transmission ofinformation depending on the respective signal power/noise power ratiodetermined in each case.
 13. A method for transmitting information asclaimed in claim 12, wherein, with a signal power/noise power ratiomeasured below a limit value, the corresponding subcarrier is notutilized for transmitting information.
 14. A method for transmittinginformation as claimed in claim 13, the method further comprising thestep of: modulating all subcarriers of the first and second transmitsignals which are not utilized for transmitting pilot signals by a samenumber of modulation levels, wherein the number of modulation levels isdetermined by a noise power/useful power ratio determined for thetransmission medium.
 15. A method for transmitting information asclaimed in claim 1, wherein the multicarrier method is implemented byone of an orthogonal frequency division multiplex transmission methodand a transmission method based on discrete multitones.
 16. A method fortransmitting information as claimed in claim 1, wherein the transmissionmedium is one of a wireless radio channel and a line-connectedtransmission channel.
 17. A method for transmitting information asclaimed in claim 16, wherein the information is transmitted via powersupply lines.
 18. A communication system for transmitting information,comprising: a first unit; a second unit; and a transmission mediumhaving particular transmission characteristics, the transmission mediumconnecting the first and second units for the transmission ofinformation between the first and second units; wherein the first unitincludes: a converter for converting, using a multicarrier method, theinformation to be transmitted into a first transmit signal having aplurality of frequency-specific subcarriers, a transmitter fortransmitting the transmit signal via the transmission medium to thesecond unit, an evaluator for determining at least one offrequency-selective amplitude-specific transmission characteristics andfrequency-selective phase-specific transmission characteristics of thetransmission medium as the transmission characteristics and averaging atleast one of the frequency-selective amplitude-specific transmissioncharacteristics and the frequency-selective phase-specific transmissioncharacteristics of adjacent subcarriers of the transmit signal fordetermining the frequency-selective transmission characteristics of thetransmission medium, and an adapter for adapting the frequency-specificsubcarriers of the transmit signal to the frequency-selectivetransmission characteristics of the transmission medium.
 19. Acommunication system as claimed in claim 18, wherein the second unitfurther includes a converter for converting, using a multicarriermethod, the information to be transmitted into a second transmit signalexhibiting a plurality of frequency-specific subcarriers, an evaluatorfor determining the frequency-selective transmission characteristics ofthe transmission medium, an adapter for adapting the frequency-specificsubcarriers of the second transmit signal to the frequency-selectivetransmission characteristics of the transmission medium which have beendetermined, and a transmitter for transmitting the second transmitsignal via the transmission medium to the first unit.
 20. Acommunication system as claimed in claim 18, wherein the evaluator isdesigned such that at least one of frequency-selectiveamplitude-specific transmission characteristics and frequency-selectivephase-specific transmission characteristics of the transmission mediumare determined as the transmission characteristics.